The present invention is directed to a tabbing device, namely, an apparatus for placing adhesive backed tabs over the lead edge of envelopes or similar flat articles such as thin packets. The invention is more particularly directed to a tabber device intended for use in small offices, in house mail departments, etc., as opposed to high volume automatic feed industrial tabbers. Still more particularly, the invention is directed to a tabber device having fail safe features and also being capable of adjustably positioning the tab which is applied such that the degree of overlap of the tab with respect to the lead surface of an article can be reliably controlled. The invention is further directed to a tabbing device which is automatically adjusted to apply tabs of different sizes.